1. Field of the Invention
This invention relates to a toner for developing an electrostatic latent image formed by electrophotography, electrostatic recording or electrostatic printing and to a toner cartridge containing the toner. The present invention is also directed to a developer containing the above toner, to a developer cartridge containing the developer, to an image forming method using the toner, and to an image forming apparatus using the toner.
2. Discussion of the Background
Various electrophotographic image forming methods have been disclosed in, for example, U.S. Pat. No. 2,297,691, Japanese Patent Publications Nos. 49-23910 and 43-24748. The methods typically include the following steps:    (1) the surface of an image bearable member such as a photoconductor is charged (charging process);    (2) the image bearable member is exposed to light to form an electrostatic latent image thereon (latent image forming process);    (3) the latent image is developed with an electrophotographic toner to form a toner image on the image bearable member (developing process);    (4) the toner image is transferred onto a receiving material (transferring process); and    (5) the toner image on the receiving material is fixed by application of heat, pressure, solvent vapor, or combination thereof to prepare a copy image (fixing process).
The method for developing electrostatic latent images is broadly classified into the following methods:    (1) a wet developing method using a liquid developer including a fine color pigment or dye dispersed in a liquid; and    (2) a dry developing method such as a cascade method, a magnetic brush method and a powder cloud method, which uses a developer (toner) including a colorant such as carbon black dispersed in a binder resin. Recently the dry developing methods are widely used.
In order to obtain a high quality image, it is important that a latent image on a latent image bearing member should be precisely developed with a toner under any given conditions. Typical characteristics of a toner required to provide high quality images include preservability (anti-blocking property), moveability, developing efficiency, transferability, chargeability and fixability.
As for fixing method in electrophotography, a heat. roller fixing method has been typically used because of good energy efficiency. However, the heat roller fixing method has a drawback because an offset phenomenon tends to occur. Namely, when a toner image is fixed, part of the toner image adheres to the heat roller and is transferred on a copy paper, resulting in formation of an undesired image.
In an attempt to solve such an offset problem, a method in which a releasing agent such as wax is included in a toner has been proposed. For, example, Japanese Laid-Open Patent Publications Nos. 51-143333, 57-148752, 58-97056 and 60-247250 disclosed the use of a solid silicone varnish, a higher fatty acid, a higher alcohol, a wax, etc. as a releasing agent.
However, when such a releasing agent is included in a toner, a filming problem tends to occur. Thus, the releasing agent is separated from the toner when developing processes are performed. The thus formed free releasing agent adheres to a photoconductor and a developing sleeve. The thus formed film of the releasing agent gradually grows as copying processes are repeated, resulting in formation of a white streak on a half tone image. Therefore, a toner including a releasing agent is required to solve the offset phenomenon and filming problem at the same time. In other words, in order to produce images having good image qualities for a long period of time, it is necessary to improve toners while paying attention to the releasing agent therein.
Currently, a need for high quality images increases more and more in the market. Satisfactory images cannot be produced by conventional toners, which typically have a volume average particle diameter of from 10 to 15 μm and, therefore, a need exists for a toner having a smaller particle diameter. However, when the particle diameter of a toner becomes small, the releasing agent included in the toner is easily separated from the toner upon being subjected to stresses, etc. In particular, when the toner is prepared by a pulverization method, stresses are predominantly exerted on the releasing agent which typically has a narrow molecular weight distribution and is brittle and, therefore, the releasing agent is apt to be present on outer surfaces of the toner particles or to be present as fine powders in the resultant toner.
Therefore, when a toner having a small particle diameter is used to produce high quality images, the filming problem is most likely to occur. To cope with this problem, Japanese Published Examined Patent Applications No. S52-3304 and No. H07-82255 propose a toner in which a polyolefin releasing agent (such as low molecular weight polyethylene, low molecular weight polypropylene) or a styrene-grafted polyolefin releasing agent is used in conjunction with a styrene resin binder resin. Since the styrene resin is poor in low temperature fixability, however, the proposed toner fails to meet with recent demand for an energy saving image forming system.
With respect to the low temperature fixability of a toner, a polyester resin is known to be suited as a binder resin therefor. However, the a releasing agent is not sufficiently dispersed in a polyester binder resin and, hence, hot offset problem is apt to occur.
Thus, in an attempt to improve low temperature fixability of toner, there are proposals to use two polyester resins having different properties as a binder resin. For example, Japanese Laid-Open Patent Publication No. S60-90344 proposes a method in which a non-linear polyester resin is used in combination with a linear polyester resin. Japanese Laid-Open Patent Publication No. S64-15755 proposes a method in which a crosslinkable polyester having a glass transition temperature (Tg) not lower than 50° C. and a softening point not higher than 200° C. is used in combination with a linear polyester resin having a softening point not higher than 150° C. and a weight average molecular weight (MW) of from 3,000 to 50,000. Japanese Laid-Open Patent Publication No. H02-82267 discloses a method in which a non-linear polyester polymer having a weight average molecular weight not less than 5,000 and a variance ratio (MW/MN) not less than 20 is used in combination with a non-linear polyester polymer having a weight average molecular weight of from 1,000 to 5,000 and a variance ratio not less than 4. Japanese Laid-Open Patent Publication No. H03-229264 proposes a method in which an organic metal compound is used in conjunction with a binder resin composed of a linear polyester resin having an acid value of from 5 to 60 and a non-linear polyester resin having an acid value less than 5. Further, Japanese Laid-Open Patent Publication No. H03-41470 proposes a method in which two kinds of saturated polyester resins having different acid values (the acid value of one polyester resin is at least 1.5 times as great as that of the other polyester resin) are used in combination.
In these methods, it is intended to achieve both low temperature fixability and high hot offset resistance by using a mixture of a non-crosslinkable resin with a crosslinkable resin. However, the blended resins have good compatibility (i.e., the resins can be mixed well with each other) and, therefore, the kneaded toner composition cannot be easily pulverized, resulting in poor productivity and high manufacturing costs.
In addition, toners in which a polyester resin having good fixability is mixed with a styrene-acrylic resin having good pulverizability are disclosed in Japanese Laid-Open Patent Publications No. S49-6931 and No. S54-11424. Since the polyester resins typically have poor compatibility with styrene-acrylic resins, however, they are unevenly dispersed in a toner when simply mixed mechanically. Therefore, a coloring agent such as carbon black and a charge controlling agent are poorly dispersed in the toner, resulting in occurrence of background stains in the toner images.